Native oxide

oxide Si nodules

Fig. 11 Film structure formed by implanting silicon with oxygen (SIMOX structure) and then annealing at a temperature greater than 1200 °C (2200 °F)

Analysis with SE consists of measuring the parameters tan(y) and cos(A) for the wavelengths of interest and then determining the thicknesses and optical constants of all the layers by regression analysis. In many cases, the optical constants are known and preprogrammed into the analysis software. Figure 12 shows the SE spectra along with the best regression fit when the nodule layer is ignored and the sample is modeled as silicon wafer/oxide/silicon/native oxide.

Fig. 12 The measured spectroscopic values for a SIMOX sample along with the calculated values using a model that ignores the silicon nodules. The arrows indicate regions of serious lack of fit.

For this fit, the thicknesses are 360.1, 211.2, and 1.8 nm for the oxide, silicon, and native oxide, respectively. Although the regression analysis curve matches the measured curve in many places, there are several locations where the misfit is noticeable, and these are indicated by arrows.

To improve the fit, the oxide can be modeled as two layers. The top layer is simply the oxide, and the bottom layer (to deal with the nodules) is a mixture of oxide and silicon. The concentration of the silicon in the mixture is one of the quantities to be determined, along with the thicknesses of the various layers. Figure 13 shows the regression fit best for this model, showing that the fit has been improved. Goodness of fit is measured by comparing the calculated curve to the measured curve and is significantly better for the curve shown in Fig. 13 than in Fig. 12. The thicknesses of the layers with this model are 32.9, 325.2, 210.9, and 1.8 nm for the mixture, oxide, silicon, and native oxide, respectively. The concentration of the silicon in the bottom oxide layer is 46%.

Fig. 13 The measured spectroscopic values for a SIMOX sample along with the calculated values using a model that includes a layer of a "mixture" consisting of oxide and silicon

Applicability. Spectroscopic ellipsometry can be used to measure layers in thicknesses ranging from a few nanometers to a few micrometers. The optical constants of many materials make them opaque in part of the spectral region and transparent in other parts of the spectrum. This feature can be used to advantage to distinguish between top layers and underlying layers. The most important feature of SE is that multilayers can readily be measured.

References cited in this section

13. W. Henderson, SOPRA, Inc. and John Woollam, J.A. Woollam Co., private communication

14. J. Vanhellemont, H E. Maes, and A. DeVeirman, J. Appl. Phys., Vol 65, 1989, p 4454

15. C. Pickering, S. Sharma, A.G. Morpeth, J.M. Keen, and A.M. Hodge, Proc. 4th International Symposium on SOI Technology and Devices, Vol 90-6, D. Schnidt, Ed., The Electrochemical Society, 1990, p 175

16. P.H. Chang and B Y. Mao, Appl. Phys. Lett., Vol 50, 1987, p 152

17. F. Ferrieu, D P. Vu, C. D'Anterroches, J.C. Oberlin, S. Maillet, and J.J. Grob, J. Appl. Phys., Vol 62, 1987, p 3458

18. S. Lynch, G.M. Crean, R. Greef, and J. Stoemonos, Appl. Surface Sci., Vol 63, 1993, p 40 Corrosion Testing of Coatings

Hermann A. Jehn and Andreas Zielonka, Research Institute for Precious Metals and Metals Chemistry

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